1. Field of the Invention
The present invention relates to an X-ray CT (computed tomography) scanner for photographing a scanogram and slice image of a subject under examination.
2. Description of the Related Art
X-ray CT scanners generally make tomograms (here called slice images) of a plurality of parallel slices of a subject under examination which are normal to the body axis of the subject. In this case, a scanogram of the subject is taken previously so as to locate the slices. That is, a scanogram which is substantially the same as a usual X-ray photograph is obtained by irradiating the subject with X-rays in synchronization with the movement of the top board of an examination couch on which the subject lies while the top board is moved through between an X-ray tube and an X-ray detector which are fixed in their respective positions. A plurality of lines representing a plurality of positions (slice positions) having a predetermined slice pitch and allowing slice images to be taken are superimposed on the scanogram on a CRT display of the X-ray CT scanner. An operator determines the position of each slice in accordance with the image on the CRT display and then enters a number corresponding to the line on the CRT display through a keyboard. The top board of the examination couch is automatically moved so that the slice to be taken of the subject is positioned directly below the X-ray tube and afterward a slice image is photographed by scanning the X-ray tube around the subject. This operation is referred to as the auto index scan.
In the auto index scan, a current of the X-ray tube (referred to as a tube current hereinafter) at the time of photographing of a slice image can be selected by switches or the like provided on a console panel. This is to secure the optimum exposure of X-rays for a portion to be photographed. Depending upon X-ray CT scanners, the X-ray tube and the X-ray detector can be shifted along a line connecting the centers thereof, and moreover the shift amount can be selected according to the size of a subject under examination. Furthermore, the tube voltage as well as the tube current may be selected.
With the conventional X-ray CT scanner, however, the so-called scan conditions, such as the tube current, the shift amount, and the tube voltage, are set empirically by an operator. Thus, an operator having little experience has to set the scan conditions on a trial and error basis. This makes the optimum setting of scan conditions very difficult. Even a well experienced operator the optimum scan conditions for each of slices. For this reason, a burden imposed on the operator in setting the scan conditions is heavy.